The present invention relates to wireless mobile radio communications and, more particularly, to beam selection strategies for non-stationary subscriber radio units.
Wireless communication networks are increasingly utilized to transfer information, such as data, voice, text or video information, among communication devices. A number of technologies and protocols have been proposed or suggested to maximize the utilization of the available bandwidth in a wireless communication network. Code division multiple access (CDMA), time division multiple access (TDMA) and frequency division multiple access (FDMA) techniques, for example, have been employed in many digital wireless communication systems to permit a large number of system users to communicate with one another.
Many wireless communication systems incorporate a mechanism to reduce co-channel interference on the forward link from a base station or cell site to a mobile receiver unit. Code division multiple access communication systems, for example, reduce co-channel interference on the forward link by modulating the information signals with Walsh orthogonal function sequences.
On the reverse link (subscriber unit-to-base station), the use of a single receive antenna in a base station in a wireless communication system causes a degradation in the received signal quality due to Rayleigh fading. As the path between the subscriber unit and the base station changes with time, due to the movement of the wireless terminal, the received signal quality vacillates, referred to as Rayleigh fading. Rayleigh fading occurs when the multipath components of a signal destructively interfere at the receive antenna causing the signal-to-noise ratio (SNR) of the composite received signal to fall below a predefined threshold.
Thus, base stations typically incorporate an antenna array having a number of spatially diverse receive antennas, to mitigate the effects of Rayleigh fading and other co-channel interference. Antenna array processing techniques mitigate the effects of multiple users by compensating for phase and delay effects. Generally, when the SNR at one receive antenna is low, the signal quality at other antennas in the array is typically satisfactory. Thus, the base station receives the transmitted signal at each antenna, compares the relative signal quality at each antenna, and dynamically selects the best signal.
Currently, there are two approaches for selecting the antenna branch with the best signal. A Switched-Beam Smart Antenna (SBSA) approach reduces interference by selecting the narrow-beam antenna branch with the best uplink (subscriber-to-base) performance. By default, the downlink (base-to-subscriber) transmissions are then over the same selected branch. SBSA is effective for traditional macrocellular base stations with small spreads of Angle-of-Arrival (AOA). For microcellular systems with low base station antenna height, however, the spread of AOA is large due to the scattering environments. Thus, SBSA is not effective in these applications.
Adaptive-Beam Smart Antenna (ABSA) approaches have been employed in environments with low base station antenna height to overcome the co-channel interference in these hostile environments. ABSA modifies the radiation pattern by using internal feedback control. The performance of ABSA techniques is strongly related to how fast the base station can estimate a better propagation channel, which is a major challenge for time-varying signals in wireless radio communications. Consequently, an ABSA approach has a high implementation cost and complexity constraints.
SBSA can be characterized as axe2x80x9chard beamxe2x80x9d selection method because SBSA always selects one and only one narrow-beam branch for transmission. ABSA can be characterized as axe2x80x9csoft beamxe2x80x9d selection method because ABSA changes its radiation pattern continuously. It is noted that both SBSA and ABSA are usually restricted to fixed objects, such as a base station, and are not suitable for non-stationary subscriber radio units. For SBSA, it is very difficult to use narrow beam transmission to acknowledge the beacon or pilot signal broadcast by base stations during the handoff process. For ABSA, the complexity and high implementation cost hinders its application to subscriber radio units.
For simplicity, subscriber radio units are usually equipped with low gain omni-directional antennas to suit the subscriber radio movement. However, with omni-directional antenna pattern characterized as non-discrimination reception, the performance degrades significantly when strong co-channel interference is present. A need therefore exists for a method and apparatus for beam selection in a non-stationary subscriber radio unit that reduces co-channel interference. A further need exists for a method and apparatus for beam selection in a non-stationary subscriber radio unit that retains the benefits of an omni-directional like antenna pattern to support handoff and better reception in scattering propagation environments.
Generally, a method and apparatus are disclosed for beam selection in a non-stationary subscriber radio unit having a multi-beam antenna array. The disclosed multi-beam antenna array acts in an omni-directional manner whenever signal-to-noise ratio (SNR) performance is sufficient, and excludes individual branches, as necessary, on the basis of SNR performance. In this manner, the present invention reduces co-channel interference while also retaining the benefits of an omni-directional like antenna pattern.
The present invention uses a multi-mode approach for the selection of appropriate antennas in the multi-beam antenna array. A given mode is established on the basis of the signal-to-noise ratio (SNR) of the received signal. Generally, if the SNR of the received signal satisfies predefined criteria, the non-stationary subscriber radio unit will operate the multi-beam antenna array in an omni-directional-like manner, by equally combining the received signal from each individual narrow-beam antenna branch. If the SNR of the received signal fails to satisfy the predefined criteria, the non-stationary subscriber radio unit will operate the multi-beam antenna array in a flex-beam manner that excludes those branches that have exhibited degraded SNR performance.
A branch ordering table is disclosed that lists the branches in the multi-beam antenna array in order of their SNR performance. A flex_counter indicates the number of branches that are currently excluded in a flex operating mode due to poor degraded SNR performance. When the non-stationary subscriber radio unit is in a flex operating mode, the received SNR of the next symbol (or frame or packet) is derived by equally combing the received signals from all individual narrow-beam branches, except the branches identified by the Flex_Counter. The transmitted signal is sent using all branches except the ones that are not included in the combined received patterns. Thus, during the flex mode, the power is redistributed into the remaining transmitting branches (those in the first part of the branch ordering table), until the SNR performance recovers.